This invention relates to a mobile communication system having a plurality of mobile stations and a base station, and in particular, to a base station in a code division multiple access (CDMA) system which executes a detecting process of phase difference between receiving signals corresponding to each of a plurality of antennas and a judging process of an arrival direction of the mobile station.
Development has been made about a mobile communication system in a code division multiple access (CDMA) system. A base station has important function as a receiving device which receives a signal from each of a plurality of mobile stations in a service are.
Further, examination has been made about an adaptive array antenna as an antenna device of the base station. In this event, the adaptive array antenna is generally composed of a plurality of omni-directional antenna devices. The omni-directional antenna device operates as a directional antenna by electrically combining receiving signals which are given from the respective antenna devices.
Under such as circumstance, it is required that the phase difference between the receiving signals is accurately detected in the adaptive array antenna. Further, it is necessary that each of the omni-directional antenna device is controlled so as to increase directional gain for an arrival direction of radio waves of the mobile station on the basis of the detected phase difference and decrease the directional gain for interference wave or disturbance wave.
Meanwhile, disclosure has been made about a radar device which realizes high distance resolution as a conventional technique in Japanese Unexamined Patent Publication No. Hei. 6-242229. The radar device has receiving time correcting means and receiving beam directional control means. The receiving time correcting means delays the receiving signals of the respective antenna devices in accordance with time correcting signals.
The receiving beam directional control means generates a time correcting signal so that arrival times of radio waves received by the respective antenna devices are equal to each other at phase coincidence plane with respect to a forming direction of an antenna beam.
Another disclosure has been made about a mobile antenna system in Japanese Unexamined Patent Publication Hei. No. 8-172312. The mobile antenna system commonly uses a local oscillator for combining same phases with a local oscillator of a monopulse circuit.
Deviation of a second intermediate frequency which generates in accordance with the respective antennas is substantially eliminated, and a phase difference signal including a phase detection error generates.
Still another disclosure has been made about a directional control antenna device in a mobile communication which improves utilizing efficiency of a communication slot or a communication channel in Japanese Unexamined Patent Publication No. Hei. 10-70502.
Such a mobile communication system is composed of an array antenna, a frequency converting means, an arrival direction estimating means and an antenna directional control means.
With this structure, the array antenna receives a signal from the base station. The frequency converting means converts a receiving signal into a signal having an intermediate frequency or a base band frequency. The arrival direction estimating means estimates an existing direction of the mobile station based upon the converted signal.
It is necessary that the conventional adaptive array antenna has a receiving portion and a signal processing portion in accordance with the respective omni-directional antenna devices. Therefore, a scale of the mobile station device becomes large, and the cost also becomes high.
To this end, it is desired that the receiving portion and the signal processing portion corresponding to the respective omni-directional antenna devices are reduced in size and cost.
Further, the conventional double super heterodyne system has been adopted in the receiving portion corresponding to each of the omni-directional antenna devices.
Accordingly, an oscillator for frequency reverse conversion (down-convert) becomes necessary in the receiving portion. The oscillator generates a signal which is locally oscillated. A phase error due to phase noise readily generates between signals which are locally oscillated in accordance with each of a plurality of antennas. In consequence, it is difficult to accurately detect the phase difference between the receiving signals corresponding to the respective omni-directional antenna devices.
Therefore, the base station, which accurately detects the phase difference between the receiving signals corresponding to the respective omni-directional antenna devices and accurately judges the arrival direction of each of the radio waves of the mobile stations, has been desirable.